1. Field of the Invention
The present invention relates to an image heating apparatus suitable for use as a fixing apparatus mounted on an electrophotographic copier or electrophotographic printer.
2. Description of the Related Art
As a fixing apparatus mounted on an electrophotographic copier or electrophotographic printer, a fixing apparatus of a film heating type is known. The fixing apparatus of the film heating type includes a heater which is made up of a resistive heating member mounted on a ceramic substrate, a cylindrical fixing film which moves in contact with the heater, and a pressure roller which forms a nip together with the heater, pinching the fixing film. A recording material carrying an unfixed toner image is heated while being pinched and conveyed at the nip, and consequently the image on the recording material is heat-fixed to the recording material by heat.
The fixing apparatus of this type has the advantage that the time required for the heater to heat up to a fixing range after the heater is turned on is short. Therefore, the printer equipped with the fixing apparatus can reduce the time required to output the first image after a print order is input (FPOT: First Print Out Time). Also, the printer has the advantage of being able to reduce power consumption while waiting on standby for a print order.
Incidentally, in the fixing apparatus described above, the operating temperature of the heater and a pressing force applied to the nip are further increased to meet market demands for higher product specifications such as improved throughput and increased image gloss. When the operating temperature of the heater or the pressing force is increased, if the temperature becomes uncontrollable due to a failure of a temperature control circuit, heater cracks could develop before a protective element, such as a thermoswitch or thermal fuse, which cuts off power supply to the heater, comes into operation. This is because when the temperature control circuit fails, the heater becomes very hot, causing a heater holder (made of resin material) which holds the heater to start melting, and if a heater holding seat surface of the heater holder melts nonuniformly in a longitudinal direction orthogonal to a recording material conveyance direction, stresses are applied to the heater by concentrating on part of the heater.
Japanese Patent Application Laid-Open No. 2007-102010 discloses a technique whereby a seat surface of the heater holder is ingeniously shaped such that a protective element will come into operation before a heater is broken.
The heater holder is provided with holes for use to dispose a temperature-measuring element such as a thermistor and a protective element, such as a thermoswitch or thermal fuse, where the temperature-measuring element measures the temperature of the heater and the protective element cuts off power supply to the heater when the heater becomes abnormally hot due to a failure of a temperature control circuit. Through the holes in the heater holder, the temperature-measuring element and protective element abut a rear surface of the heater (surface opposite to that surface of the heater which faces the nip) at an appropriate pressure. Incidentally, hole portions in the heater holder provide air layers, i.e., heat insulating layers. There are gaps around the temperature-measuring element and the protective element inserted in the hole portions, and the gaps serve as heat insulating portions. Therefore, when abnormal heat is generated by the heater, heat tends to build up around the heat insulating portions, causing the heater holder to melt faster. On the other hand, portions away from the hole portions of the holder melt later than the hole portions. Consequently, it has become clear that the hole portions, on which stresses are concentrated, are subject to a shearing force, resulting in heater cracks.